1. Field of the Invention
The invention relates generally to backfill material for ground bed anodes and particularly to a low resistance backfill material which is a mixture of carbonaceous materials and a surfactant.
2. Description of the Prior Art
The practice of utilizing deep well anode beds to prevent corrosion and rapid deterioration of subsurface metallic structures is an effective method of increasing the life of such structures.
Under various conditions, corrosion of subsurface metallic structures has been caused by galvanic action due to the creation of anodic and cathodic areas on the metallic structures. It was found that corrosion occurred at the anodic area of the structure at which areas a current flow was established from the metallic structure into the surrounding soil and water which acted as an electrolytic medium. However, the cathodic areas of the subsurface structures at which the flow of current was directed or collected from the surrounding medium to the metallic structure were found to remain relatively free from corrosive action.
In order to prevent the corrosive action at anodic areas along the subsurface structures, it was determined that various forms of electrodes could be placed in the ground in the general vicinity of the subsurface metallic structure and an electric current was impressed thereon which flowed into the surrounding soil to substantially all areas of the subsurface structure. Since the impressed current was greater than the galvanic current, such impressed current overpowered the galvanic current on the subsurface structure so that substantially the entire surface area of the structure became cathodic. In this manner, the electrode acted as an anode which became subject to electrochemical attack and the subsurface metallic structure was protected from such corrosive action as its surface was cathodic. Such a process became known in the field as "cathodic protection."
Although cathodic protection has been widely accepted, its effectiveness is a direct function of the effective life of the electrode and the impressed current used to establish current flow. Early electrodes consisted of utilizing metallic pipes, rails, beams and various scraps which were buried in the ground adjacent the subsurface structure to be protected and supplying an electric current to such members. However, as such electrodes were subject to corrosive effects, their maximum effective life was dependent upon the weight of the material, the amount of current used, and the soil conditions including soil acidity and moisture content.
In use such electrodes tended to separate along areas of localized corrosion and therefore portions of the electrode were removed or separated from the current supply. Such localized corrosion substantially decreased the effective life of the electrode resulting in an effective life range of between 4 to 8 years, again dependent upon the various conditions mentioned above. Therefore, various carbon and graphite electrodes have also been widely used in order to provide continuous cathodic protection. However, it is necessary to replace the expended electrodes with new ones, a process which adds to the expense of maintaining such a system.
From the above, it is apparent that in order to increase the economical operation of a cathodic protection system, it is desirable to utilize electrodes having a very low rate of consumption in terms of pounds of electrode per ampere per year. Further, the cost of electrode replacement is an important consideration.
As discussed above, the rate of consumption of the anode material was subject to various factors including possible localized separation. In this respect, it was noted that the rate of consumption was dependent upon the current density at the interface between the anode and the medium surrounding the same. In order to provide or establish a more uniform flow of current from the anode to the electrolyte, use was made of a uniformly low-resistance backfill material to completely surround the anode. Materials including granular or pulverized carbon substances such as calcined coke, graphite and the like were used not only to provide a uniformly low-resistance medium, but also to effectively decrease the electrical resistance between the anode and the electrolyte. As discussed in U.S. Pat. No. 2,553,654 to Heise, the use of such backfill permitted a significantly increased current density along the anode.
At high rates of current flow, however, and especially when water has seeped into the backfill material, hydrogen gas may be formed along the anode. If the gas cannot escape, pockets of hydrogen are formed along the anode and act as insulating barriers. Therefore, the type of backfill used and configuration of the anode bed should be such as to insure that any gas created by electrochemical action may freely escape from the anode bed.
Further, as discussed in applicant's prior U.S. Pat. No. 3,725,669, the backfill material for an earth anode bed ordinarily is placed in the bed by fluidizing the backfill with water and pumping the same into the anode bed. Such fluidizing backfill material also permits ease of anode replacement, again as described in applicant's aforementioned patent. Therefore, it is advantageous to provide a backfill material which is not only very low in resistance to current flow, but one which is easily pumped with water and which is quickly settleable therefrom to provide a dense backfill medium for an emplaced anode.
Heretofore, although carbonaceous backfill materials have been used to provide a low resistance medium to insure even current flow from an earth anode and permit increased current diversities along the surface of the anode, the placement of such backfill in the anode bed has not been as efficient as is possible.
Other examples of the prior art include U.S. Pat. No. 3,857,991 to Higashimura et al. and applicant's product bulletin LORESCO.